Today, satellite systems provide us with a great deal of valuable information. For example, earth orbiting satellites have proven to be a great asset in the gathering of intelligence information, weather information, and in the implementation of ground based positioning systems. In particular, satellites have proven to be effective in the imaging of surface features on the earth.
In typical imaging applications, a satellite is implemented as an orbiting telescope. Typically included in the telescope is some sort of pointing device, such as a mirror. The pointing device is typically assembled with various moving components (such as bearings) to provide for the movement thereof. Typically, the mirror scans the surface of the earth in a set pattern, and a telescope which receives images from the mirror, provides data to imaging apparatus to assemble a composite image.
However, as there is a demand for better quality images, such as closer views of surface features, the performance of the pointing device must be improved. One source of error in such a pointing device includes the error in the actual pointing. Such error is typically a result of limitations in the moving components.
In many high-precision devices, operation of rotating components is frequently limited by bearing error, or “wobble.” Examples of rotating components include an optical imaging device on a satellite or a hard disk drive (HDD) within a data storage unit. Typically, performance requirements for rotating components are limited by the bearing manufacturing techniques. For example, improved performance typically requires that the bearings include surfaces that are as smooth as possible, which, at a minimum typically means costly manufacturing techniques. However, even with improved manufacturing, there is no way to eliminate bearing related variability altogether.
In radial bearings, variability may occur in the shapes or dimensions of the surfaces, and inconsistencies in the materials. Examples of radial bearings include ball bearings, roller bearings, tapered roller bearings, magnetic bearings, air-filled bearings and others. In roller bearings, variability may be seen in the surfaces such as the rolling bodies (such as the balls, rollers, tapered rollers, etc, . . . ), as well as in the raceways of the inner and outer rings. In non-roller bearings, variability may materialize in the magnetic field strength or fluid distribution. The variability inherent in these bearings causes error, which is often referred to as “run-out,” or “point run-out.” Run-out has two components, one being repeatable and the other non-repeatable.
In roller bearings it is generally assumed that the “repeatable point run-out” (RPR) is caused by inconsistencies of the raceway. That is because typically, for any particular orientation of an inner ring in relation to an outer ring, the inconsistencies will generally line up and remain static. The Non-Repeatable Point Run-Out (NRPR) is usually attributed to the rolling elements as the rolling elements generally align themselves differently even when the inner and outer rings are orientated in a particular way. Regardless of the cause, the errors observed in any bearing will have a predictable component and an apparently random component. These are referred to herein as RPR and NRPR respectively.
One way to overcome manufacturing limitations in a bearing is to measure the effect of the variability and then compensate for the effect. Various systems have been used to measure the error of revolving shafts, and in some cases, such as in the machine tool industry, even incorporate eddy current proximity sensors. One known method for compensation from limitations is the use of a control system to adjust positioning of components of the device based upon the measured error. Another known method is to correct error in software. One useful source of information on this topic is the Internet site for the Machine Dynamic Research Laboratory at Penn State University.
The effect of repeatable point run-out (RPR) may be reduced, at least initially, as RPR can be measured and compensated for prior to service of the bearing. However, over time, RPR changes due to wear of the bearing. The wear leads to degradation in performance, unless the RPR can be measured and compensated for during service. Therefore, measuring or understanding the effect of the point run-out, whether repeatable or non-repeatable, can be more important than having a bearing produced at the limits of manufacturing precision.
Research has been conducted to assess RPR. However, this research has not evaluated the effect of RPR in compound systems (i.e. systems including bearings operating along various axes). More specifically, current techniques provide for evaluation of run-out in a bearing. These systems do not account for angular error in a device mounted to bearings rotating along an azimuth axis and an elevation axis.
Examples of research into bearing error include graduate student theses published by Penn State University, and available at the Internet website of the Machine Dynamics Research Laboratory thereof. One example is entitled “Use and Calibration of Ultra-Precision Axes of Rotation with Nanometer Level Metrology,” by Bob Grejda. This thesis presents a spindle calibration device designed to quantify nanometer level error motions of ultraprecision axes of rotation. The device has the ability to determine error motions in various directions for a spindle under test.
Existing research, as is presently known, falls short of determining angular error that arises in a pointing device from aggregation of the RPR errors, where the pointing device is mounted in an assembly that includes bearings operating on separate axes. Further, a system has not been devised to provide for accurate measurement of the position of a pointing device used on board a spacecraft, (also referred to as a space-based platform).
What is needed is a system that can be incorporated into a device having a rotating component, in order to measure the non-repeatable point run-out (NRPR) as it occurs, and changes in repeatable point run-out (RPR). The system should provide for high quality data over the service life of the component. The system should be amenable to being deployed in a spacecraft, and should therefore be capable of having a service life measured in years.